1. Technical Field
The present invention relates, generally, to methods and apparatus for controlling the burnoff rate of a consumable wire electrode in an arc welding process, and more particularly, to an adaptive controller for supplying a pulsed voltage reference train to a voltage controlled arc welding power source such that the wire burnoff rate follows the apparent wire feed rate.
2. Background Art and Technical Problems
Hand-held welding tools configured to cooperate with a power source and a controller are generally well known. In the gas metal arc welding (GMAW) process, a consumable wire electrode is fed to the weld site by a wire feeding mechanism configured to cooperate with a hand-held tool. In such systems, a first electrical contact extends from the power source to the workpiece such that the workpiece functions as a cathode. A second electrical contact extends from the power source to the consumable wire electrode so that the wire functions as an anode.
The power source supplies current to the wire, which current traverses the gap between the workpiece and the wire, forming a plasma column therebetween. This plasma column is sometimes referred to as an "arc"; the voltage difference between the workpiece and the wire electrode is referred to as the arc voltage. A shielding gas, for example 2% oxygen and 98% argon, is fed through the wire feed conduit to the welding site to shield the arc from the atmosphere.
At arc current and voltage levels above a predetermined threshold, spheres of molten metal are melted off the end of the wire electrode, travel through the arc plasma column and are deposited onto a melt puddle at the weld site. The rate at which molten metal is exhausted from the wire electrode is a function of power dissipated at the arc, where power is equal to the product of voltage and current. Inasmuch as a relatively constant voltage is maintained across the arc, the rate of wire consumption is generally a function of arc current.
The quality of the weld is largely determined by the amount of heat generated within the plasma column. To avoid excessive workpiece deformation, it is desirable to minimize arc current while maintaining sufficient power dissipation at the end of the wire electrode to ensure an adequate supply of molten metal to the weld site. Thus, a frequency modulated pulsed current train is supplied to the arc, through the consumable wire, by the power source.
The power source generates the pulse train in response to a voltage reference signal supplied thereto from the controller. The controller modulates the frequency of the reference signal supplied to the power source, thereby controlling the frequency of the current pulse train and, hence, the amount of heat generated at the wire. See, for example, Cook U.S. Pat. No. 3,896,287 issued Jul. 22, 1975, and Yamamoto et al. U.S. Pat. No. 4,409,465 issued Oct. 11, 1983.
Presently known GMAW control systems are unsatisfactory in several regards. In a typical GMAW process employing pulsed current control, five principal parameters influence the burnoff rate at the weld site: (1) frequency of the current pulse train delivered to the wire electrode; (2) peak current amplitude; (3) background current amplitude; (4) peak pulse width (and, hence, background pulse width); and (5) wire feed rate. Welding systems which control wire feed rate or, alternatively, systems predicated on a constant feed rate, typically require a dedicated controller. Consequently, a particular welding tool may only be used with a particular power source, reducing flexibility and efficiency.
Presently known control systems also do not provide satisfactory transient process control prior to steady state operation, i.e., during arc initiation.